The invention relates to a keyboard musical instrument, and more particularly to a method and apparatus for digitally generating musical notes in an electronic musical instrument.
There are two known approaches to digitally generating musical notes. The first approach encompasses musical instruments which generate musical tones from one or more continuous varying inputs which are subsequently shaped by essentially linear networks into a musical output. The second approach encompasses musical instruments which generate musical tones from a sequence of discrete data samples. These tones are converted from processed data samples into analog signals by digital to analog converters, and are scaled or amplified to the desired output level. The instruments that use this later approach are referred to as digital synthesizers.
U.S. Pat. No. 4,602,545 by David Starkey issued Jul. 29, 1986 entitled DIGITAL SIGNAL GENERATOR FOR MUSICAL NOTES, describes a type of digital synthesizer. The inventor of this patented digital synthesizer is also the inventor of the present invention, and the disclosure of U.S. Pat. No. 4,602,545 is hereby incorporated by reference. This known digital synthesizer uses waveforms which are stored in memory (either ROM or loaded RAM) as wavetables. The wavetables contain samples of the waveform and are adequate to define the harmonic structure, which is defined beforehand and selected by a musical stop switch or other similar device. This known synthesizer reads the stored samples in a time sequence according to the pitch of the note selected on a keyboard or other similar device. Further, this digital synthesizer uses an interpolator to interpolate the value of the stored waveform between samples, which reduces the amount of granularity noise present in the audio output because of using discrete samples of the waveform. As mentioned in this patent, the interpolator gives the reduced granularity of larger memories and larger stored sample sizes, without the cost of additional memory or the need of more lines in the address bus.
As good as this digital signal generator is, it still has some limitations. The granularity or smoothness of the waveform as it changes to subsequent points of the waveform is reduced, but it is not removed. Another problem results from the multiplying digital to analog convertor used to perform the envelope shaping after the interpolator section has determined the waveform. The multiplying digital to analog converters use a lookup table in memory which is controlled by the keyboard stop and the key played signals to determine the attack and decay portions of the envelope shape. The X.sup.2 piecewise approximation of the attack and decay portions of the envelope are sometimes difficult approximations to make from the selections available in the lookup table because of the parabolic nature of the function. This often results in noticable increases in noise and distortion of the musical output.
It is an object of this invention to provide a digital signal generator of the digital synthesizer type that has a reduced granularity of the interpolated waveform.
It is another object of this invention to provide a digital signal generator with an envelope shaping system that generates less noise and distortion.
It is another object of this invention to provide a digital signal generator that exponentially approximates the attack and decay portions of each note envelope.
It is another object of this invention to provide a digital signal generator that uses digital scaling of the interpolated waveforms to perform amplitude variation and envelope shaping.